1. Field of the Invention
The present invention relates to an ink-jet printhead and a method of manufacturing the same. More particularly, the present invention relates to an ink-jet printhead and a method of manufacturing the same that is able to obtain a substantially flat nozzle plate, thereby extending a lifespan of the printhead.
2. Description of the Related Art
In general, ink-jet printheads are devices for printing a predetermined image, color or black, by ejecting a small volume droplet of ink at a desired position on a recording sheet. Ink-jet printheads are generally categorized into two types depending on which ink ejection mechanism is used. A first type is a thermal ink-jet printhead, in which a heat source is employed to form and expand a bubble in ink to cause an ink droplet to be ejected due to an expansive force of the formed bubble. A second type is a piezoelectric ink-jet printhead, in which an ink droplet is ejected by a pressure applied to the ink due to a deformation of a piezoelectric element.
An ink droplet ejection mechanism of a thermal ink-jet printhead will now be explained in detail. When a current pulse is supplied to a heater, which includes a heating resistor, the heater generates heat and ink near the heater is instantaneously heated to approximately 300° C., thereby boiling the ink. The boiling of the ink causes bubbles to be generated, expand and exert pressure on the ink filling an ink chamber. As a result, ink around a nozzle is ejected from the ink chamber in droplet form through the nozzle.
A thermal ink-jet printhead is classified into a top-shooting type, a side-shooting type, and a back-shooting type, depending on a growth direction of a bubble and an ejection direction of an ink droplet. In a top-shooting type printhead, a bubble grows in the same direction in which an ink droplet is ejected. In a side-shooting type of printhead, a bubble grows in a direction perpendicular to a direction in which an ink droplet is ejected. In a back-shooting type of printhead, a bubble grows in a direction opposite to a direction in which an ink droplet is ejected.
An ink-jet printhead using the thermal driving method should satisfy the following requirements. First, manufacturing of the ink-jet printheads should be simple, costs should be low, and should facilitate mass production thereof. Second, in order to obtain a high-quality image, cross talk between adjacent nozzles should be suppressed while a distance between adjacent nozzles should be narrow; that is, in order to increase dots per inch (DPI), a plurality of nozzles should be densely positioned. Third, in order to perform a high-speed printing operation, a period in which the ink chamber is refilled with ink after being ejected from the ink chamber should be as short as possible and the cooling of heated ink and heater should be performed quickly to increase a driving frequency.
FIG. 1A illustrates an exploded perspective view of a conventional thermal ink-jet printhead. FIG. 1B illustrates a cross-sectional view for explaining a process of ejecting an ink droplet in the conventional thermal ink-jet printhead of FIG. 1A.
Referring to FIGS. 1A and 1B, the conventional thermal ink-jet printhead includes a substrate 10, an ink chamber 26, which is formed on the substrate 10 and stores ink therein, partition walls 14, which define the ink chamber 26, a heater 12, which is disposed within the ink chamber 26, a nozzle 16, through which an ink droplet 29′ is ejected, and a nozzle plate 18, through which the nozzle 16 is formed. In operation, a current pulse is supplied to the heater 12 to generate heat, such that ink 29 filled in the ink chamber 26 is heated, thereby generating a bubble 28. The generated bubble 28 continuously expands such that a pressure is applied to the ink 29 filled in the ink chamber 26, thereby ejecting the ink droplet 29′ out of the printhead through the nozzle 16. Subsequently, ink 29 from a manifold 22 is introduced into the ink chamber 26 through an ink channel 24. Resultantly, the ink chamber 26 is refilled with ink 29.
To manufacture the conventional top-shooting type ink-jet printhead constructed as above, the nozzle plate 18, in which the nozzle 16 is formed, is required to be separately manufactured from the substrate 10, on which the ink chamber 26 and the ink channel 24 are formed. Subsequently, the nozzle plate 18 and the substrate 10 are required to be bonded together. Thus, the manufacturing process is complicated and misalignment may occur during the step of bonding the nozzle plate 18 to the substrate 10. In addition, when the nozzle plate 18 is bonded to the substrate 10, it is very difficult to ensure that bonded portions therebetween have a uniform thickness. In addition, because the ink chamber 26, the ink channel 24, and the manifold 22 are disposed on a same level, an increase in the number of nozzles 16 per unit area, i.e., nozzle density, is limited. As a result, it is difficult to realize an ink-jet printhead having high printing speed and high resolution.
To solve the problems of the conventional ink-jet printhead, various types of ink-jet printheads have been suggested recently. One example of an attempt to solve these problems is a conventional monolithic ink-jet printhead shown in FIG. 2.
Referring to FIG. 2, a hemispherical ink chamber 32 is formed in an upper portion of a silicon substrate 30, and a manifold 36 is formed in a lower portion of the substrate 30. An ink channel 34 passes through the ink chamber 32 and is interposed between the ink chamber 32 and the manifold 36 to provide flow communication between the ink chamber 32 and the manifold 36. A plurality of material layers 41, 42, and 43 are stacked on the substrate 30 to form a nozzle plate 40. The nozzle plate 40 is integrally formed with the substrate 30. A nozzle 47 is formed in the nozzle plate 40 at a position corresponding to a central portion of the ink chamber 32. A heater 45 is disposed around the nozzle 47 and is connected to a conductor 46. A nozzle guide 44 is formed along an outer peripheral surface of the nozzle 47 and extends toward the ink chamber 32. Heat generated by the heater 45 is transmitted to ink 48 filled in the ink chamber 32 through an insulating layer, i.e., the lowest material layer, 41. Accordingly, the ink 48 is boiled to generate bubbles 49. The generated bubbles 49 are expanded to exert a pressure on the ink 48 filled in the ink chamber 32. Therefore, the ink 48 is ejected in the form of a droplet 48′ through the nozzle 47. Subsequently, ink 48 is introduced through the ink channel 34 from the manifold 36 to refill the ink chamber 32 with ink 48.
In this conventional ink-jet printhead constructed as above, since the silicon substrate 30 is integrally formed with the nozzle plate 40, the manufacturing process is simplified and misalignment may be avoided. Furthermore, since the nozzle 47, the ink channel 34, and the manifold 36 are vertically arranged, the conventional ink-jet printhead of FIG. 2 may achieve higher nozzle density than the conventional ink-jet printhead of FIGS. 1A and 1B.
However, in the conventional ink-jet printhead shown in FIG. 2, the material layers 41, 42, and 43, which are formed around the heater 45, are made of a material having a low thermal conductivity, such as oxide or nitride, to provide electrical insulation. Accordingly, it requires a significant amount of time to sufficiently cool the heater 45, which has generated heat to eject the ink 48, the ink 48 filled in the ink chamber 32, and the nozzle guide 44 to initial states thereof, thereby failing to sufficiently increase an operating frequency.
The material layers 41, 42, and 43 constituting the nozzle plate 40 in this conventional ink-jet printhead are formed using chemical vapor deposition (CVD). It is difficult to form thick material layers using CVD. As a result, since the nozzle plate 40 has a relatively small thickness of approximately 5 μm, the nozzle 47 cannot be long enough to adequately eject the ink droplet 48′. If the nozzle 47 is short, the linearity of the ejected ink droplet 48′ decreases. Further, since it is possible that a meniscus of the ink 48 does not remain in the nozzle 47, but penetrates into the ink chamber 32 after the ink droplet 48′ is ejected, a stable high speed printing operation cannot be ensured. While the nozzle guide 44 is formed along the outer peripheral surface of the nozzle 47 in an effort to solve these problems, if the nozzle guide 44 is too long, it complicates formation of the ink chamber 32 by etching the substrate 30 and limits the expansion of the bubbles 49. Because of the nozzle guide 44, there is a limitation in achieving a nozzle having a sufficient length.
Additionally, an outlet of the nozzle 47 in the conventional ink-jet printhead does not have a sharp edge but a round edge, which becomes wider toward the outside of the printhead. Hence, the ejection characteristics of the ink droplet 48′ decrease and an outer surface of the nozzle plate 40 is easily wet with the ink 48.